The way they look at it is that if one partner 'withholds sex' from the other, it makes the probability of adultery higher for the 'offended' partner. If that does happen, they may feel compelled to make you share the 'blame'.
It doesn't make sense to me now, surely as adults we should be able to accept the responsibility for our own behaviour.
I also wonder if insinuating themselves into your private bedroom activities is perversely tittillating for some elders.
Love, Scully